This invention relates to an apparatus for transferring a pattern of a first object onto a second object and, more particularly, to an apparatus for transferring a semiconductor circuit pattern of a reticle or mask onto a wafer, during processes for manufacturing semiconductor circuit devices such as integrated circuits (ICs), large scaled integrated circuits (LSIs), very large scaled integrated circuits (VLSIs), etc.
A known type of pattern transfer apparatus for use in the manufacture of semiconductor devices is illustrated in FIG. 1. In this pattern transfer apparatus, an illumination unit 1 for illuminating a pattern formed on a pattern bearing surface 4a of a reticle 4 carried by a reticle stage 5, is disposed above the reticle 4, and an alignment unit 2 for achieving alignment between the reticle 4 and a wafer 7 carried by a wafer stage 8 is disposed between the reticle 4 and the illumination unit 1. A projection lens 6 is employed to project the pattern of the reticle 4 onto the wafer 7. The illumination unit 1 includes a masking member 1a located at a position optically conjugate with the pattern bearing surface 4a of the reticle 4. The masking member 1a is effective to block the light so that an area on the reticle 4 which should not be exposed is shielded against the light. The masking member 1a has a light-transmitting window whose area can be variably set relative to the pattern bearing surface 4a of the reticle 4.
The alignment unit 2 comprises an optical system and a predetermined relation should be maintained between this optical system and the pattern bearing surface 4a of the reticle 4. For example, in a case where the alignment unit 2 is arranged to detect any positional deviation between the reticle 4 and wafer 7 by scanning alignment marks formed on the reticle 4 and wafer 7 with a laser beam emitted from the alignment unit 2 and where the alignment between the reticle 4 and wafer 7 is achieved on the basis of the result of detection, the laser beam emerging from the alignment unit 2 has to be focused on the pattern bearing surface 4a of the reticle 4.
If, on the other hand, the thickness of the reticle carried by the reticle stage 5 is changed, the optical path length from the illumination unit 1 or the alignment unit 2 to the pattern bearing surface 4a of the reticle 4 changes accordingly. As the result, the conjugate relation between the masking member 1a and the pattern bearing surface 4a of the reticle 4 can not be maintained. Further, the laser beam emerging from the alignment unit 2 would not be focused on the pattern bearing surface 4a. Such changes in the optical path length must be compensated for.
It is now assumed that the reticle 4 is replaced by another reticle 4b having a thickness T which is greater than the thickness t.sub.1 of the reticle 4 by an amount t.sub.2. In such case, the illumination unit 1, alignment unit 2 and a mirror 3 have to be moved upwardly by an amount suitable for correcting the optical path length through an amount corresponding to the increment t.sub.2 in the thickness of the reticle. The amount of movement d can be given by the following equation: EQU d=t.sub.2 (1-1/n) (1)
wherein n is the refractive index of each of the reticles 4 and 4b.
An ordinary reticle has a thickness t.sub.1 =2.3 mm. It is now assumed that the reticle 4b substituted for the reticle 4 has a thickness T (=t.sub.1 +t.sub.2)=6 mm (=2.3 mm+3.7 mm) and a refractive index n=1.516. From equation (1), it follows that the amount of movement d of each of the illumination unit 1, alignment unit 2 and mirror 3 is approx. 1.26 mm.
Each of the illumination unit 1 and the alignment unit 2 must be disposed in a determined relation, with respect to the distance, with the pattern bearing surface of the reticle in order to maintain a predetermined positional relation such as focusing or conjugate relation. For this reason, practically the illumination unit 1 and the alignment unit 2 are positioned, during assembly of the pattern transfer apparatus, relative to the reticle stage 5 while taking into account the thickness of the reticle which thickness is usually t.sub.1 (=2.3 mm).
However, there exist commercially available reticles, although not so many, having thicknesses different from the thickness t.sub.1, e.g. the aforementioned T (=6 mm). If use of such reticles is desired, the illumination unit 1 and the alignment unit 2 have to be adjusted again relative to the reticle stage 5. Actually, however, such adjustment after the transfer apparatus has been assembled is not practicable because it requires extraordinarily difficult and complicated operations to accurately move the illumination unit 1 and the alignment unit 2 through a substantial distance such as 1.26 mm where the thickness is T (=6 mm).